Light Detection And Ranging (LiDAR) surveys are one of our Woodland Heritage Services
What is LiDAR?
Hillshaded model of ground surface
Hillshaded LiDAR image of woodland
LiDAR has the potential to show many archaeological features previously hidden from aerial reconnaissance by woodland cover (Figure 1).
LiDAR uses an aircraft mounted ‘eye-safe’ laser which rapidly scans the terrain below (typically at a rate of between 20,000 and 50,000 pulses a second). As the aircraft travels forward, the laser scans left and right whilst producing the rapid pulses of energy. For every individual laser pulse, some of the energy is reflected back to the aircraft and the time taken for the reflection to be detected is recorded. As light travels at a known and constant speed, by combining the reflection times with very sophisticated GPS and navigational computers on the aircraft, a three-dimensional spatial reference can be determined for the surface below each laser pulse. By overlapping parallel flight paths, it is possible to survey large areas, producing millions of 3-dimensional data points known as ‘point clouds’. The density of the point clouds equates to the number of laser pulses per square metre of terrain surface and the resolution of the survey.
By importing these point clouds into mapping software, it is possible to produce detailed 3-dimensional models of the terrain surveyed. This model is used to create a ‘hillshaded’ image within the computer which represents the data as a two-dimensional picture by emphasizing changes in relief. (Figure 2).
'Seeing through' woodland
However, unlike other aerial surveys such as photography (Figure 3), LiDAR has the potential to ‘see through’ woodland and produce 3-dimensional models of the entire forest structure - see below:
3D model of woodland
3D model of ground surface
Example of raw data point cloud
When flown over woodland, the terrain surface below the aircraft is effectively porous and for every single laser pulse, some of the energy will be reflected back from the upper canopy whilst some may be reflected back from lower in the canopy, the understory vegetation, or possibly the true ground surface. Thus, a whole series of reflections can therefore be produced for every single laser pulse.
Most LiDAR detectors record the times for the first and last reflections to be detected that relate to the top of the canopy and potentially the woodland floor respectively. Some newer systems are able to record hundreds of intermediate reflection times for every laser pulse and have the potential to build complex
If a deciduous woodland is flown during winter, the vast majority of the laser energy can pass through the canopy to the floor. Computer processing can filter the LiDAR data to effectively remove the reflections from the canopy and allow a ground surface to be modelled, potentially revealing archaeological earthworks that have never previously been recorded in aerial photographs
The survey produces rapid, highly accurate maps for use in the field, whilst larger surveys also allow features to be examined in the landscape setting. LiDAR is particularly well suited to linear features and has shown very subtle earthworks that are often difficult to see on the ground. Under optimum conditions (well-spaced broadleaf woodland with minimal understorey vegetation), the method can show smaller features such as charcoal platforms. Nonetheless, even in stands of conifer where laser penetration is restricted, some larger earthworks may still be evident in the final images.
What are the limitations?
This technique is not necessarily the answer to every archaeologist’s dreams, as it does have limitations:
- There are many laser configuration and flight variables that can alter the resolution of the survey. All of these must be considered prior to commissioning a survey to ensure optimum conditions for the area under investigation.
- Despite surveying in winter, not all canopy and understory vegetation types are equally porous to the laser energy and where little light naturally penetrates to the forest floor a LiDAR survey may be of little use.
- Whilst under optimum conditions the method will show more discrete features such as charcoal platforms, it is inevitable that not all features of similar or smaller size will be discernible in the resulting images.
- Vegetation removal and hillshading are necessary parts of the data processing. However, both can either remove or hide features of interest. An understanding of the processes involved in producing these models and images are therefore essential.
- LiDAR does not distinguish between modern, archaeological or vegetation produced features in the hillshaded images. Features of potential archaeological interest can result from modern drain clearance, woodland thinning residue left by a roadside or bracken which has died over winter but when draped over a wire fence resembles an earth bank. Whilst it may be a long-term project, some ground-truthing will be necessary.
- Sherwood Pines
- Harlow Wood
- Old Park Wood
- New Forest Remembers project
Surveys of woodland using LiDAR (PDF-1631K)
Developments in remote sensing technology to reveal the historic environment by ‘seeing through’ the woodland canopy and its application in mapping, with direct relevance to forest management.
Past Landscapes beneath the trees, using lidar in woodlands.
On 8th September 2010 the Archaeology Service of Gloucestershire County Council and the Forestry Commission jointly hosted a successful conference at the University of Gloucestershire's Park Campus in Cheltenham.
Papers from this conference.
Crutchley, S. and Crow, P. (2009). The light fantastic: Using airborne laser scanning in archaeological survey. English Heritage, Swindon.
Crow, P., et al. (2007). Woodland vegetation and its implications for archaeological survey using LiDAR. Forestry. 80 (3) pp 241-252.
Devereux, B.J. et al. (2005). The potential of airborne lidar for detection of archaeological features under woodland canopies. Antiquity Vol. 79, 305 pp 648-660.